[Tex/LaTex] includegraphics: How to check if PDF/PNG are transparent

graphicspdfpdftextransparency

I am trying to generate PDF/A-1 compliant documents, in which transparency is not allowed. I would like to check, if included PDF and PNG images have transparency.

Is there a TeX way to check this? Otherwise, what is the proper way to change the \includegraphics command to call external tools to check this?

Edit: External tools for checking transparency:

For PNGs, ImageMagik's identify does the job nicely. It displays [s]rgba when there is an alpha channel (otherwise it's rgb):

# identify -format '%[channels]' Transparent.png 
srgba

Even better:

# identify -format %A Transparent.png 
True

For PDFs, this is more complicated (see eg here): There are as graphical style attributes fill opacity (/ca) or stroke opacities (/CA) below a value of 1.0, or there is a soft mask (\SMask), or there is a transparency page group defined (/S /Transparency). grep expression:

grep -aE -e '/[Cc][Aa] +0?\.[0-9]' -e '/SMask' -e '/S /Transparency' *.pdf

I bet it is somehow possible to check with gs, but I did not find it so far.

Best Answer

I would use ghostscript to remove the transparency once the pdf is generated, using

gs -o converted.pdf -sDEVICE=pdfwrite -dHaveTransparency=false input_file.pdf

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[Tex/LaTex] \includegraphics PDF, color problem

Hey I had the same problem with my Lyx docment using pdflatex. I simply copied the line from Martin Heller, \pdfpageattr {/Group << /S /Transparency /I true /CS /DeviceRGB>>} and now my images containing transparency are correctly displayed in the final PDF:)

[Tex/LaTex] PNG (with transparent background) and \pdfximage \pdfrefximage

The obvious problem is that the generated number is not 2 for the second image, as a simple

\showthe\pdflastximage

put just after

\pdfximage height 28.0mm             {my_other_pic.png}

will show:

> 3.
l.18 \showthe\pdflastximage

This is probably due to the fact that one more picture object is included for "masking" the one with transparent background.

There is another primitive, \pdflastximage that can (and should) be used for these situations: it is a read-only integer that is loaded with the number assigned to the last object included with \pdfximage. However you can't say

\newcommand{\LOGOi}{\pdfrefximage\pdflastximage}

because this would always refer to the last image. Some lower level trickery is needed:

\pdfximage width 20.0mm height 8.0mm {png_transparent_bg.png} 
\edef\LOGOi{\pdfrefximage\the\pdflastximage\relax}

\pdfximage height 28.0mm             {my_other_pic.png} 
\edef\LOGOii{\pdfrefximage\the\pdflastximage\relax}

\pdfximage height 20.0mm             {my_cool_pic.png} 
\edef\LOGOiii{\pdfrefximage\the\pdflastximage\relax}

Each \edef will expand \the\pdflastximage to the current value; in your case, the expansions of \LOGOi, \LOGOii and \LOGOiii will be

> \LOGOi=macro:
->\pdfrefximage 1\relax .

> \LOGOii=macro:
->\pdfrefximage 3\relax .

> \LOGOiii=macro:
->\pdfrefximage 4\relax .

as required for a correct later usage of the images.

It's quite unclear why you don't want to use the higher level \includegraphics for this. If you're afraid that multiply used pictures are included multiple times in the PDF file, don't be.

In case you've inherited some document where these low level tricks are used, just comment out the lines and change them according to this model

%\pdfximage width 20.0mm height 8.0mm {png_transparent_bg.png} 
%\edef\LOGOi{\pdfrefximage\the\pdflastximage\relax}
%\pdfximage height 28.0mm             {my_other_pic.png} 
%\edef\LOGOii{\pdfrefximage\the\pdflastximage\relax}
%\pdfximage height 20.0mm             {my_cool_pic.png} 
%\edef\LOGOiii{\pdfrefximage\the\pdflastximage\relax}

\usepackage{graphicx}
\newcommand\LOGOi{\includegraphics[width 20.0mm, height 8.0mm]{png_transparent_bg.png}}
\newcommand{\LOGOii}{\includegraphics[height 28.0mm]{my_other_pic.png}}
\newcommand{\LOGOiii}{\includegraphics[height 20.0mm]{my_cool_pic.png}}

Best Answer

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[Tex/LaTex] \includegraphics PDF, color problem

[Tex/LaTex] PNG (with transparent background) and \pdfximage \pdfrefximage

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